In fanfiction, Anakin joining the Dark Side often has complex motives. It could be due to his fear of losing Padmé. The Dark Side might seem to offer him a way to save her, so he succumbs to its power.
Perhaps a fanfic that delves into the time when Anakin was on the cusp of turning to the dark side. Padme could sense the change in him and tries to prevent it with the help of Luke and Leia from the future who somehow travel back in time. This would create a complex web of relationships and a race against time to save Anakin from the dark side.
Fanfiction usually develops Anakin's character by showing his relationships in a new light. His bond with Palpatine could be shown as more complex, with Palpatine slowly manipulating Anakin over a long period, preying on his weaknesses like his desire for power to save Padmé. This makes his turn seem more gradual and believable.
One great Dark Anakin fanfiction could be about his internal struggle. It might show how the dark side slowly creeps in as he faces difficult decisions in the Clone Wars. For example, a story could focus on a mission where he's tempted to use excessive force to achieve victory, and how that starts to change his mindset towards the dark side.
The fanfic could be set during the Clone Wars. Anakin's dark side starts to emerge due to the stress and manipulation. Padme notices the changes in him. Meanwhile, Luke and Leia are prophesied to be the ones who can save the galaxy from the dark side that Anakin is falling into. It could show how their fates are intertwined even before they are born, with Padme trying to protect the future of her children and Anakin.
A great fanfiction idea could be that Dark Anakin kidnaps Padme (not in a malicious way, but more out of his distorted sense of protecting her). He takes her to a remote planet where they are forced to confront their feelings for each other without the interference of the Jedi or the Senate. This would be a very intense and emotional story.
A common theme is the fall to the dark side. It explores how Anakin gradually becomes Dark Anakin, his internal struggles, and the events that lead to his transformation. Another theme is redemption. Some fanfics try to show a way for Dark Anakin to come back from the dark side.
Betrayal is also a big theme. He betrays the Jedi Order, his friends, and ultimately himself. His relationship with Obi - Wan, for example, is often explored in these fanfictions as a relationship that is shattered by his turn to the dark side. And there's the theme of internal conflict. Anakin is constantly at war with himself, his good side fighting against the allure of the dark side, which is a central aspect in many of these stories.
Fanfiction.net is also a good place to look. It has been around for a long time and has a large community of Star Wars fans who write fanfictions. Just type in 'Dark Anakin and Padme' in the search bar, and you'll get a list of relevant stories. However, the quality may vary, so you might need to read a few to find the really good ones.